The present invention relates to a receiving technique for spectrum spreading communication in which a spectrum-spread signal by direct spreading is received.
A direct spreading-spectrum spreading communication system using a spectrum spreading (SS) by means of direct spreading (DS) has a feature that interference is difficult to occur and communication capacity is large and so forth, and the system has been studied and developed as a communication system for communication and so forth of an automobile telephone.
Referring to FIG. 8, the direct spreading-spectrum spreading communication system will be explained. This direct spreading-spectrum spreading communication system is substantially the same as a disclosure as a prior art in JP-A-90222/1994, and has a transmitting device A for spectrum spreading communication and a receiving device B for spectrum spreading communication.
The transmitting device A for spectrum spreading communication has a spreading code generator 4' for generating a spreading code such as a pseudo noise signal (PN code) as a spreading code on a transmitting side. A spectrum spreading section 2' applies spectrum-spreading by means of direct spreading to an information signal using this spreading code on the transmitting side, and outputs a spectrum-spread signal. The spectrum spreading section 2' is a multiplier for multiplying the spreading signal by the information signal. A modulating section 3' phase-modulates a carrier having a predetermined carrier frequency with the spectrum-spread signal (base band signal), and outputs a phase-modulated carrier. An antenna 5' transmits the phase-modulated carrier as a radio signal. In this manner, the radio signal carries the spectrum-spread signal as the base band signal.
The receiving device B for spectrum spreading communication has an antenna 6' for receiving the radio signal as a receiving signal. A synchronizing circuit 7' has a spreading code same as the spreading code on the transmitting side which is generated by the spreading code generator 4' of the transmitting device A for spectrum spreading communication, as a spreading code on a receiving side, synchronizes the spreading code on the receiving side with the spreading code of the transmitting side of the receiving signal, and outputs a synchronized spreading code to a spectrum inverse spreading section 8'. This spectrum inverse spreading section 8' applies spectrum-inverse-spreading to the receiving signal with the synchronized spreading code, and outputs a spectrum-inverse-spread signal. This spectrum inverse spreading section 8' is a multiplier for multiplying the receiving signal by the spreading code. A demodulating section 9' demodulates the above-mentioned information signal from the spectrum-inverse-spread signal.
Here, in order to establish the synchronization in the synchronizing circuit 7', a point is searched where phases in the receiving signal of the spreading code on the receiving side to the spreading code on the transmitting side coincide with each other, and timing thereof has to be controlled within a predetermined range.
Referring to FIG. 9, a conventional synchronizing circuit is shown, which is used as the synchronizing circuit 7' in FIG. 8. The synchronizing circuit shown in the figure includes a frequency converter that has a local oscillator 1 for outputting a local oscillating signal having a predetermined oscillating frequency and a mixer 11 for mixing a receiving signal with the local oscillating signal and outputting a mixed signal. At this time, the mixer 11 outputs the above-described mixed signal having a frequency that is equal to the difference between the predetermined local oscillating frequency and a carrier frequency of the receiving signal. Since the predetermined local oscillating frequency is previously selected so that the difference between this predetermined local oscillating frequency and the carrier frequency is equal to a frequency of a base band signal (spectrum-spread signal), the mixer 11 outputs the above-described mixed signal having a frequency that is equal to the frequency of the base band signal (spectrum-spread signal).
An LPF (Low Pass Filter) 2 outputs the base band signal (spectrum-spread signal) from this mixed signal. A sample hole circuit 3 sample-holes the spectrum-spread signal and sends a correlator 4 the spectrum-spread signal that is under sample-held condition.
The correlator 4 is constructed of a matched filter and multiplies codes for one period of spreading codes on a transmitting side of a spectrum-spread signal by codes for one period of the previously given and above-mentioned spreading codes on a receiving side, and outputs the summation of results of the multiplication. This summation becomes to be maximum in case that timing of the spreading code on the transmitting side of the spectrum-spread signal and timing of the spreading codes on the receiving side coincide with each other. In this manner, the correlator 4 performs the above-described multiplication chip by chip corresponding to the spreading codes on the receiving side for one period, and outputs the summation of the results of the multiplication chip by chip.
A synchronization detecting circuit 5 detects a matched pulse indicating that the above-described summation is maximum, and outputs the above-described spreading codes on the receiving side at time when detecting the matched pulse, as synchronized spreading codes, to the spectrum inverse spreading section 81.
Referring to FIG. 10, the correlator 4 in FIG. 9 is shown. The correlator 4 shown in the figure is constructed of a matched filter, and has a shift register capable of storing codes for one chip of the spectrum-spread signal obtained as a base band signal. A coefficient generator 4b generates codes for one chip of the spreading codes on the receiving side. A multiplier 4d multiplies codes for one chip stored in the shift register 4a by the codes for one chip of the spreading codes on the receiving side, and outputs results of the multiplication. An adder 4c outputs the summation of the results of the multiplication. This summation becomes to be maximum in case that timing of the spreading codes on the receiving side from the coefficient generator 4b and timing of the spreading code on the transmitting side of the spectrum-spread signal coincide with each other. At this time, the adder 4c outputs the above-mentioned matched pulse indicating that the above-described summation is maximum.
However, in a method for synchronization using the synchronization circuit in FIG. 9, since correlation of phase shifts of the spreading codes is not zero, a pseudo peak occurs at timing other than a peak of proper correlation. Accordingly, in case of combining a plurality of paths with different timings by means of a RAKE combination, since it is impossible to discriminate between a peak due to multi-paths and a pseudo peak occurring due to the non-zero correlation, there has been a task that characteristic after the RAKE combination deteriorates.